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New Baghouse, Recuperator, Boost Cupola Efficiency

Jan. 16, 2009
Acme Foundry's updated cupola has an efficiency rating of 3.8 tph per 1,000 scfm of equivalent blast rate which is estimated to be a 30% increase in melting efficiency.
Acme Foundry's updated cupola has an efficiency rating of 3.8 tph per 1,000 scfm of equivalent blast rate which is estimated to be a 30% increase in melting efficiency.

It was obvious the cupola furnace baghouse at the Acme Foundry in Coffeyville, KS, needed replacement. It wasn’t MACT compliant and acid had eaten away at the 35-year-old structure, causing leaks and reducing its overall effectiveness. Only constant maintenance kept it operational. Tom Tatman, Acme Foundry president, said it was common for at least two employees to spend a couple of hours every shift performing maintenance on the baghouse. In addition to those labor costs, he said the foundry was spending $25-30,000 annually on replacement parts and new bags.

For Acme Foundry, which produces highly cored, gray iron castings, replacing its cupola baghouse was the last step necessary to achieve full MACT compliance for particulate emissions control. When also factoring in the annual maintenance costs, the decision to replace the baghouse was an easy one.

But, a closer look revealed an opportunity to impact the operation beyond those initial objectives. By installing an entirely new emissions control system with a hot-blast recuperator and a twostage, dry gas cooling system, it would be possible to increase production efficiency and significantly reduce overall operating costs. Acme chose a system designed by Kuttner North America to increase the efficiency of the cupola, and consume less coke, oxygen and natural gas, and generate a quick return on the investment. Kuttner engineers also identified potential improvements for improved chemistry control and increased operating flexibility for the melt operation.

When designing a state-of-the-art emissions control system, using an inefficient furnace as the starting point isn’t recommended. As part of the initial planning process, Kuttner engineers examined all aspects of the Acme Foundry cupola operation — scrap properties, required melt rates, production schedule, iron chemistry and temperature requirements, fugitive emissions, and cupola gas combustion stability. This was followed by the evaluation of a variety of retrofit scenarios that included improvements in gas handling system controls, the use of recuperative hot blast, dry gas cooling, and dust collection.

Kuttner determined that gaining furnace efficiency was critical to delivering the results Acme was seeking. Acme was using an unlined, cold-blast cupola that had a very high coke rate and required an oxygen injection rate of more than 2.5%. Without a refractory lining, the heat loss from the furnace to the water cooling system was a significant factor in the furnace’s low thermal efficiency.

Kuttner installed a 1,050°F hot-blast recuperator, redesigned the water-cooled tuyeres and added a new melt zone refractory lining, making the combustion and melting processes more efficient.

Now, the Acme furnace is melting with an efficiency of about a 3.8 tph per 1,000 scfm of equivalent blast rate. This represents about a 30% increase in overall melt efficiency making it one of North America’s most efficient cupolas.

Coke and oxygen usage dropped substantially, while allowing the furnace to melt at Acme’s desired rate of 16 tph and using a blast rate 25% less than that of the old furnace.

“Our metal-coke ratio went from 8 to 1 to 12 to 1, decreasing coke usage by 60-80 pounds per charge,” according to Tatman. “We’re using almost 20-25% less coke, while our oxygen use has dropped off substantially, by 80-85%. We feel good about the cost savings and it helps us maintain our competitiveness within the marketplace.”

Although Acme’s furnace is using less coke per charge, Tatman says the charge make-up (rail, automotive scrap, pig iron, and alloys) remains the same. This was significant to maintaining the foundry’s reputation for quality products.

Acme ships approximately 20,000 tons/year of gray iron castings to manufacturers in the fluid power, transmission machinery and petro-chemical industries. It produces up to five grades of cupola-based gray iron every day, and carbon/silicon control, and the management of residuals, such as chrome, copper and nickel are critical to quality. Nothing leaves the facility that does not meet customer specifications or pass the foundry’s strict, ISO 9000 quality control program.

“We did not see a decrease in carbon at all, and the charge make ups that we’ve been running for the last 20 years remained the same,” said Tatman. “Our charge materials are different than everyone else’s and we did not want to change their composition. We’ve been able to save money on coke, yet keep the quality of our iron the same.”

The new refractory lining has also improved chemistry control because spout metal get s hot quicker. When the cupol a is turned off, it remains hot for 30-45 minutes. When it’s turned back on, it returns to the required operating temperature quickly, contributing to the reduction of coke and oxygen.

Acme’s new furnace is a long-campaign cupola, which can run for about seven weeks before the refractory lining needs repair. Half of the 9-in. lining in the melt zone remains after a five-week campaign — resulting in extremely low maintenance costs for the cupola lining.

These improvements greatly reduced furnace operating costs, while at the same time, created less heat and gas load for the new gas handling system. As a result, Kuttner could design a system using a recuperator, two-stage gas cooler, and baghouse that were much smaller, and less expensive, than if those cupola improvements were not made.

The cupola isn’t the only part of the system where Kuttner’s expertise and technology led to increased efficiency. Kuttner’s advanced baghouse design, equipped with extremely compact Luehr filter bags, is one-third the size of the Hartzell baghouse that it replaced.

Despite the small footprint, more bags and a larger filtration surface area were designed into the new baghouse, resulting in a very low air-to-cloth ratio, in the range of 1.2 to 1.5:1, depending on operating conditions, making filtration very efficient and leading to a very long bag life. With the “water-free,” two-stage gas-cooling system, Kuttner expects the Aramid fiber bags to last 5-10 years.

The closed-loop system uses a two-pass, cross-flow recuperator, a two-pass cross-flow aftercooler, and a Luehr gas cooler prior to filtration. This gas conditioning is critical to dust collection and allows for optimal effectiveness in particulate emissions control. In addition, the layout of this equipment allows for waste-heat recovery, providing the potential for Acme to capture and reuse the heat somewhere in its facility.

Although Acme is a “synthetic minor” source under the MACT standards established by the U.S. EPA, Acme required that the new baghouse meet, or exceed, the existing major source particulate limit of 0.006 grains per dry standard cubic feet (gr/dscf).

Compliance tests conducted 20 days after start-up yielded 0.0019 gr/dscf — far exceeding MACT requirements. In addition, carbon monoxide emission rates fell below 50 ppm because of the upgraded afterburner system and new controls. The Acme cupola is also compliant with the 1,300°F minimum afterburner temperature limit required by the U.S. EPA under the new MACT standard.

Kuttner worked closely with Acme to coordinate the installation plan. There were some particularly tight areas that were needed for equipment placement, and Acme had to remain in production throughout the majority of the construction. Much of the new equipment was installed without affecting production.

Kuttner engineers evaluated every component within the previous system and were even able to find a use for the old water-spray quencher that had been used to cool the gas before it hit the bag house. It now functions as a drop out for fly ash to ensure the abrasive material doesn’t continue through the recuperator or first-stage gas cooling system. This arrangement will substantially extend tube life in the hot-blast recuperator.

The cupola furnace and the entire gas-handling system is controlled by Allen Bradley PLCs and a Dell PC-based RSView Human Machine Interface (HMI). All critical operating parameters (e.g., temperatures, pressures, and flow rates) are quickly accessible to the operator using intuitive graphic displays and drop-down menus. In addition, the HMI displays messages, warnings and alarms, and logs all messages based on preset parameters. If the system detects that one of these parameters is out of range, or there is a warning or fault, it gives a verbal description of what’s happening within the system. All of the parameters are recorded every few milliseconds and stored for use in historical trending, aiding diagnosis, troubleshooting, and fine tuning.

“It was important for us to automate the control of the system,” said Tatman. “We wanted to make it as easy as possible for our operators to monitor the activity in the system and have the ability to react should anything happen.”

Although these types of controls are standard within the industry, Kuttner added value by properly applying the technology within the system. Kuttner’s team of engineers has decades of experience building and retrofitting cupola melting and emissioncontrol systems. Their understanding of the cupola furnace, thermodynamics, combustion processes, and environmental regulations allows them to configure systems where everything is reacting properly and provide turnkey solutions.

“What’s impressive about Kuttner’s solution and the use of the hot-blast recuperator is that we could significantly increase our production quickly,” said Tatman. “We are running 16 tons an hour because our molding systems are set up specifically for that tonnage. We’ve added enough potential capacity to our melting capabilities to help increase our tonnage to 25 tons an hour. Both in the short term and long term, we see a payback in our future.”